forall x is an open access introductory textbook in formal logic. It covers translation, proofs, and formal semantics for sentential and predicate logic.

It is available under the Creative Commons license. There is no charge to use the book either as a text for a course or for self-directed study, and you are welcome to revise it to fit your needs. It is available as a free download, and you can make hardcopies without owing anything.